A typical method of that kind in which the paper web generally passes in a four-color first-form and perfecting procedure through eight printing mechanisms, downstream of each of which is provided a respective drier, provides that the solvent used to put the ink into a suitably liquid form, generally toluene, is evaporated by means of hot air which flows through the respective drier and serves as an energy carrier, at least to such an extent that, on the freshly printed side, the paper web can be passed over guide or direction-changing rollers without ink being transferred on to same, or that the paper web can be printed upon with another ink in a downstream printing mechanism, without involving smearing and smudging of the ink first applied.
In that respect, for reasons relating to safety engineering, it is not possible to exceed a maximum solvent concentration of 50% of what is known as the `lower explosion limit`, often referred to by the abbreviation LEL, in which case the value of that LEL is dependent on the temperature in the drier. For air which has an oxygen content of about 21%, 50% LEL at 20.degree. C. corresponds to about 21 g/m.sup.3 while at 80.degree. C. it is only about 16 g/m.sup.3.
Further important parameters in regard to the drying methods in question are the speed at which the paper web passes through the printing mechanisms and the driers, and the length that the individual driers must be so that an adequate drying action can be achieved.
An adequate drying action in that respect not only requires fulfilment of the two conditions referred to above, but also involves a sufficiently low residual solvent content in the finished printed product as the solvent residue quantity which is still contained therein is not to exceed certain specified limits, both for health reasons and also in regard to complying generally with environmental pollution conditions, and also for the reason that otherwise, when folded printed products are laid one upon the other or when a printed paper web which has not yet been cut into pieces is wound on to an intermediate storage roll, the various sheets or layers could stick together, so that the entire production would be rendered useless.
Modern production processes seek to achieve ever increasing paper web speeds and larger and larger paper web widths, but nonetheless the lengths of the driers and the amount of hot air which can be passed through each drier per unit of time cannot be increased just as may be desired. It therefore becomes extremely difficult under those conditions to ensure a drying action which is `adequate` in the above-indicated sense.